1. Field of the Invention
The present invention generally relates to chromatographic analysis and, in particular, to methods for classifying samples by utilizing chemometrics analysis for interpreting chromatographic data obtained under locked conditions.
2. Description of the Related Art
Gas chromatography is a method for analyzing a sample of interest, which may include one or more analytes, to qualitatively determine the identity of the analytes as well as to quantitatively determine the concentration of each of the analytes. This analysis also may include comparing the sample of interest to previously analyzed samples for the purpose of classifying the sample of interest. Gas chromatography typically involves a series of steps, including: sample collection, sample preparation, sample introduction into a chromatographic or separation column, chromatographic separation of the sample into individual analytes, detection of those analytes, and data acquisition and reduction. As is well known, the step of data reduction, which enables the identification of the analytes present in the sample, can be a complex task, primarily due to the multivariate nature of the acquired data.
Chemometrics is a general term applied to the use of statistics and mathematics to extract meaningful information from multivariate data. In a representative implementation, statistical algorithms may be employed to help one reduce large amounts of data into a small number of salient descriptors. Chemometrics typically is applied for one or more of the following primary purposes: (1) to interrogate data efficiently (trend analysis, correlation of results, outlier identification, identification of significant variables, etc.); (2) to track data or a measurement function on an on-going basis (control charting, limit detection); (3) to quantify results based on multivariate response (complex calibration/response functions), and; (4) to reduce complex (e.g., multivariate or multiple response) measurements into classes of like results (classification).
In the context of chromatography, chemometrics may be utilized to provide pattern recognition functionality for interpreting acquired chromatographic data and tends to be most effective when variability in retention times of samples eluting through the separation column of a gas chromatograph is minimized. Heretofore, variability in retention times typically has been addressed by applying a post-analysis adjustment to acquired chromatographic data, such as by mathematically xe2x80x9caligningxe2x80x9d the chromatographic time scales of chromatograms relating to multiple samples. However, post-analysis mathematical approaches for xe2x80x9caligningxe2x80x9d chromatograms include inherent weaknesses, such as requiring reference peaks or patterns to be present in the data for automatic adjustment to work consistently. This may require adding compounds to each sample (internal standards) and also may lead to errors because samples of unknown composition can vary widely in the response or ratio of response of peaks, thereby potentially leading to incorrect assignment of reference peaks or patterns.
Additionally, typical post-analysis mathematical adjustment of acquired data can not compensate for changes in the order of elution of peaks that can result from operation of GC systems under unlocked conditions. Thus, even if reference peaks are identified and the time axis of the chromatographic data is adjusted for an overall improvement, the retention times of several key individual peaks may still be incorrect (possibly even worse than before the adjustment), potentially resulting in the incorrect classification of the resulting xe2x80x9cadjustedxe2x80x9d patterns.
Therefore, there exists a need for improved methods which address these and other shortcomings of the prior art.
Briefly described, the present invention relates to methods for analyzing samples by utilizing chemometric analysis for interpreting chromatographic data obtained under locked conditions. A preferred method comprises the steps of: (1) providing a locking GC system which includes a column operated at a column head pressure; (2) adjusting the column head pressure of the locking GC system so that a column void time of the column in the locking GC system, when a known analyte is eluted therethrough, is matched with the column void time of the column in a reference GC system, when the known analyte is eluted therethrough; (3) analyzing the sample with the locking GC system so that chromatographic data corresponding to the sample is compiled, and; (4) performing chemometric analysis on the chromatographic data.
Alternatively, the column head pressure of the locking GC system may be adjusted so that a retention time of the known analyte in the column of the locking GC system is matched with the retention time of the known analyte in the column of the reference GC system.
In accordance. with another aspect of the present invention, an alternative embodiment for chromatographically analyzing a sample comprises the steps of: (1) receiving chromatographic data from a locking GC system; and (2) performing chemometric analysis on the chromatographic data.
Other objects, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such objects, features, and advantages be included herein within the scope of the present invention, as defined in the appended claims.